1. Field of the Invention
The invention relates to a process for producing a gallium nitride crystal substrate, and a gallium nitride crystal substrate.
2. Prior Art
GaN compound semiconductors, such as gallium nitride (GaN), indium gallium nitride (InGaN), and gallium aluminum nitride (GaAlN), have drawn attention as materials for blue light-emitting diodes (LEDs) and laser diodes (LDs). Further, by virtue of good heat resistance and environmental resistance, GaN compound semiconductors have been used in the optical devices, as well as in the development of elements for electronic devices utilizing these features.
In the GaN compound semiconductors, however, it is difficult to grow bulk crystal, and, for this reason, GaN substrates, which can be put to practical use, have not been produced yet. Sapphire is a substrate for the growth of GaN which is presently extensively put to practical use, and it is common practice to epitaxially grow GaN on a single crystal sapphire substrate, for example, by metal-organic vapor phase epitaxy (MOVPE).
The sapphire substrate is different from GaN in lattice constant, and, thus, when GaN is grown directly on the sapphire this problem, Japanese Patent Laid-Open No. 188983/1988 discloses a method wherein a buffer layer of AlN or GaN is once grown at a low temperature on the sapphire substrate, for reducing strain of lattice and GaN is then grown on the buffer layer.
Even in the case of the growth of GaN using the low-temperature grown buffer layer, however, a difference in lattice constant between the substrate and the crystal occurs, and, consequently, GaN has numerous defects which are expected to be an obstacle to the production of GaN-base LDs. Further, due to the difference in a coefficient of linear expansion between the sapphire substrate and GaN, warpage occurs in the substrate after epitaxy, and, in the worst case, the substrate is disadvantageously cracked.
For this reason, the development of GaN bulk substrates has been eagerly desired.
Although the growth of a large bulk GaN crystal is very difficult, a method has recently been proposed wherein a thick film of GaN is heteroepitaxially grown on a substrate, for example, by HVPE and the substrate is then removed to provide a free standing substrate of GaN.
By the way, at the present time, any technique for separating GaN grown on the sapphire substrate by etching has not been developed. Although an attempt to mechanically remove the sapphire substrate by polishing has also been made, the warpage of the substrate is increased in the process of polishing and the probability of cracking of the substrate is high. For this reason, this method has not been put to practical use.
Here Jpn. J. Appl. Phys. Vol. 38 (1999) Pt. 2, No. 3A reports a method wherein, after the growth of a thick GaN film on a sapphire substrate by HVPE, laser pulses are applied to separate only the GaN film. However, this method also is likely to cause cracking of the substrate.
Further, Japanese Patent Laid-Open No. 12900/2000 discloses a method using an easily removable substrate. In this method, a thick GaN film is grown on a GaAs substrate by HVPE and the GaAs substrate is then removed by etching. According to this method, a large GaN substrate can be prepared in a relatively high yield. This method, however, has a problem that the GaAs substrate is disadvantageously decomposed during the growth of the GaN crystal and arsenic (As) is included as an impurity into the GaN.
Selective growth using a patterned mask is effective for reducing the defect density of epitaxially grown GaN, and, for example, Japanese Patent Laid-Open No, 312971/1998 discloses a technique for this. Since, however, there is no method for easily separating the substrate, the above technique cannot be effectively utilized in the production of a free standing substrate of GaN.
Accordingly, it is an object of the invention to solve the problems of the prior art and to provide a process for producing, in a simple manner, a gallium nitride crystal substrate, which has low defect density and has not been significantly contaminated with impurities, and a gallium nitride crystal substrate produced by the production process.
According to the first feature of the invention, a process for producing a gallium nitride crystal substrate, comprises the steps of:
depositing a metal film on a starting substrate, which is any one of a single crystal sapphire substrate, a substrate comprising a single crystal gallium nitride film grown on a sapphire substrate, and a single crystal semiconductor substrate;
depositing a gallium nitride film on the metal film to form a laminate substrate, and
removing the starting substrate from the laminate substrate with the gallium nitride film deposited thereon to prepare a free standing gallium nitride crystal substrate.
According to the second feature of the invention, a process for producing a gallium nitride crystal substrate, comprises the steps of:
depositing a metal film on a starting substrate, which is any one of a single crystal sapphire substrate, a substrate comprising a single crystal gallium nitride film grown on a sapphire substrate, and a single crystal semiconductor substrate;
forming a mask region and a gallium nitride selective growth region formed of a patterned mask material on the metal film;
depositing, using the selective growth region as an origin, a gallium nitride film on the selective growth region and the mask region to form a laminate substrate; and
removing the starting substrate from the laminate substrate with the gallium nitride film deposited thereon to prepare a free standing gallium nitride crystal substrate.
According to the third feature of the invention, a process for producing a gallium nitride crystal substrate, comprises the steps of:
depositing a metal film on a starting substrate, which is any one of a single crystal sapphire substrate, a substrate comprising a single crystal gallium nitride film grown on a sapphire substrate, and a single crystal semiconductor substrate;
depositing a gallium nitride film on the metal film;
forming a mask region and a gallium nitride selective growth region formed of a patterned mask material on the gallium nitride film;
again depositing, using the selective growth region as an origin, a gallium nitride film on the selective growth region and the mask region to form a laminate substrate; and
removing the starting substrate from the laminate substrate with the gallium nitride film again deposited thereon to prepare a free standing gallium nitride crystal substrate.
According to the fourth feature of the invention, a process for producing a gallium nitride crystal substrate, comprises the steps of:
forming a mask region and a gallium nitride selective growth region formed of a patterned mask material an a starting substrate, which is any one of a single crystal sapphire substrate, a substrate comprising a single crystal gallium nitride film grown on a sapphire substrate, and a single crystal semiconductor substrate;
depositing, using the selective growth region as an origin, a gallium nitride film on the selective growth region and the mask region;
forming a metal film on the gallium nitride film;
again depositing a gallium nitride film on the metal film to form a laminate substrate; and
removing the starting substrate from the laminate substrate with the gallium nitride film again deposited thereon to prepare a free standing gallium nitride crystal substrate.
The production processes according to the first to fourth features of the invention having the above respective constructions may further comprise the steps of;
depositing a metal film on a starting substrate, which is the free standing gallium nitride crystal substrate;
depositing a gallium nitride film on the metal film to form a laminate substrate; and
removing the starting substrate from the laminate substrate with the gallium nitride film deposited thereon to prepare a free standing gallium nitride crystal substrate.
The production processes according to the first to fourth features of the invention having the above respective constructions may farther comprise the steps of:
depositing a metal film on a starting substrate, which is the free standing gallium nitride crystal substrate;
forming a mask region and a gallium nitride selective growth region formed of a patterned mask material on the metal film;
depositing, using the selective growth region as an origin, a gallium nitride film on the selective growth region and the mask region to form a laminate substrate; and
removing the starting substrate from the laminate substrate with the gallium nitride film deposited thereon to prepare a free standing gallium nitride crystal substrate.
The production processes according to the first to fourth features of the invention having the above respective constructions may further comprise the steps of:
depositing a metal film on a starting substrate, which is the free standing gallium nitride crystal substrate;
depositing a gallium nitride film on the metal film;
forming a mask region and a gallium nitride selective growth region formed of a patterned mask material on the gallium nitride film;
again depositing, using the selective growth region as an origin, a gallium nitride film on the selective growth region and the mask region to form a laminate substrate; and
removing the starting substrate from the laminate substrate with the gallium nitride film again deposited thereon.
The production processes according to the first to fourth features of the invention having the above respective constructions may further comprise the steps of:
depositing a metal film on a starting substrate, which is the free standing gallium nitride crystal substrate;
forming a mask region and a gallium nitride selective growth region formed of a patterned mask material on the metal film;
depositing, using the selective growth region as an origin, a gallium nitride film on the selective growth region and the mask region to form a laminate substrate; and
removing the starting substrate from the laminate substrate with the gallium nitride film deposited thereon to prepare a free standing gallium nitride crystal substrate.
The production processes according to the first to fourth features of the invention having the above respective constructions may further comprise the steps of:
depositing a metal film on a starting substrate, which is the free standing gallium nitride crystal substrate;
depositing a gallium nitride film on the metal film;
forming a mask region and a gallium nitride selective growth region formed of a patterned mask material on the gallium nitride film;
again depositing, using the selective growth region as an origin, a gallium nitride film on the selective growth region and the mask region to form a laminate substrate; and
removing the starting substrate from the laminate substrate with the gallium nitride film again deposited thereon to prepare a free standing gallium nitride crystal substrate.
The production processes according to the first to fourth features of the invention having the above respective constructions may further comprise the steps of:
forming a mask region and a gallium nitride selective growth region formed of a patterned mask material on a starting substrate which is the free standing gallium nitride crystal substrate;
depositing, using the selective growth region as an origin, a gallium nitride film on the selective growth region and the mask region;
forming a metal film on the gallium nitride film;
again depositing a gallium nitride film on the metal film to form a laminate substrate; and
removing the starting substrate from the laminate substrate with the gallium nitride film again deposited thereon to prepare a free standing gallium nitride crystal substrate.
In the above production processes, the metal film is preferably a metal film having C-axis orientation.
In the above production processes, the metal film is preferably formed of a member selected from the group consisting of aluminum, gold, silver, copper, platinum, iron, nickel, titanium, zirconium, and hafnium and alloys containing any one of said metals.
In the above production processes, preferably, the metal film has a thickness in the range of 10 to 1000 nm.
In the above production processes, preferably, the gallium nitride is deposited to a thickness of not less than 50 xcexcm.
In the above production processes, aluminum nitride may be deposited on the metal film, followed by the deposition of gallium nitride on the aluminum nitride.
In the above production processes, preferably, a part or the whole of the step of depositing gallium nitride is carried out by HVPE.
In the above production processes, preferably, the step of removing the starting substrate is the step of separating the gallium nitride film from the substrate through the metal
In the above production processes, preferably, the gallium nitride film is separated from the starting substrate by etching the metal film.
According to the invention, a metal film is deposited on a starting substrate, which is any one of a single crystal sapphire substrate, a substrate comprising a single crystal gallium nitride film grown on a sapphire substrate, and a single crystal semiconductor substrate, and a gallium nitride film is deposited on the metal film to form a laminate substrate. By virtue of the above construction, after the growth of the gallium nitride film, the single crystal of gallium nitride can be easily separated from the starting substrate.
The separation of the grown single crystal of gallium nitride from the starting substrate can also be facilitated by the adoption of a construction wherein a mask region and a gallium nitride selective growth region formed of a patterned mask material are formed on a metal film, a gallium nitride film is deposited, using the selective growth region as an origin, on the selective growth region and the mask region to form a laminate substrate, and a starting substrate is formed from the laminate substrate.
Further, the separation of the grown single crystal of gallium nitride from the starting substrate can also be facilitated by the adoption of a construction wherein a gallium nitride film is deposited on a metal film, a mask region and a gallium nitride selective growth region formed of a patterned mask material are formed on the gallium nitride film, and a gallium nitride film is again deposited, using the selective growth region as an origin, on the selective growth region and the mask region to form a laminate substrate.
Furthermore, the separation of the grown single crystal of gallium nitride from the starting substrate can also be facilitated by the adoption of a construction wherein a mask region and a gallium nitride selective growth region formed of a patterned mask material are formed on a starting substrate, a gallium nitride film is deposited, using the selective growth region as an origin, on the selective growth region and the mask region, a metal film is formed on the gallium nitride film and a gallium nitride film is again deposited on the metal film to form a laminate substrate.
In particular, not only the facilitation of the separation of the grown single crystal of gallium nitride from the starting substrate but also a further reduction in crystal defects can be realized by the adoption of a construction wherein a metal film is deposited on a starting substrate, which is a free standing gallium nitride crystal substrate, and a gallium nitride film is deposited on the metal film to form a laminate substrate with the gallium nitride film deposited thereon.
In particular, not only the facilitation of the separation of the grown single crystal of gallium nitride from the starting substrate but also a further reduction in crystal defects can be realized by the adoption of a construction wherein a metal film is deposited on a starting substrate, which is a free standing gallium nitride crystal substrate, a mask region and a gallium nitride selective growth region formed of a patterned mask material are formed on the metal film, and a gallium nitride film is deposited, using the selective growth region as an origin, on the selective growth region and the mask region to form a laminate substrate with the gallium nitride film deposited thereon.
In particular, not only the facilitation of the separation of the grown single crystal of gallium nitride from the starting substrate but also a further reduction in crystal defects can be realized by the adoption of a construction wherein a metal film is deposited on a starting substrate, which is the free standing gallium nitride crystal substrate, a gallium nitride film is deposited on the metal film, a mask region and a gallium nitride selective growth region formed of a patterned mask material are formed on the gallium nitride film, a gallium nitride film is then again deposited, using the selective growth region as an origin, on the selective growth region and the mask region to form a laminate substrate with the gallium nitride film again deposited thereon.
In particular, not only the facilitation of the separation of the grown single crystal of gallium nitride from the starting substrate but also a further reduction in crystal defects can be realized by the adoption of a construction wherein a mask region and a gallium nitride selective growth region formed of a patterned mask material are formed on a starting substrate, which is a free standing gallium nitride crystal substrate, a gallium nitride film is deposited, using the selective growth region as an origin, on the selective growth region and the mask region, a metal film is formed on the gallium nitride film, and a gallium nitride film is again deposited on the metal film to form a laminate substrate.